1. Field of the Invention
The present invention relates to a method for regenerating a filter of a coal gasification system being equipped with a filter for removing char (unburned matter) in exhaust gas at an exhaust gas passage communicating with exhaust gas source including a coal gasification furnace of the coal gasification system.
2. Description of the Related Art
In an integrated coal gasification combined power-generating unit having a coal gasification furnace, a porous filter for removing char (unburned matter) remaining in gas produced in the coal gasification furnace. In the normal operation of the coal gasification furnace, char captured by the porous filter is backwashed with compressed nitrogen and the backwashed char is recovered into a hopper and returned to the coal gasification furnace.
The life of a porous filter is determined by the increase of airflow resistance on an element inside the porous filter. The porous filters which are used in the backwash treatment as described above usually last about two years. Thus, there are various methods for regenerating the porous filters which have reached the end of the product life.
For example, JP8-309151A published on Nov. 26, 1996 (hereinafter referred to as Publication 1) teaches a regeneration method of a porous filter which removes dust in exhaust gas which is applied to a diesel engine and not a coal gasification furnace.
Publication 1 discloses a method of regenerating the porous filter, wherein exhaust gas is introduced into a denitration-catalytic ceramic filter house, the filtered exhaust gas is circulated to the upstream of the filter, the exhaust gas is adjusted to contain 5-20 vol. % oxygen (hereinafter vol. % is used to describe volume percentage) and at 80-400° C., the adjusted exhaust gas is passed gradually through the filter, thus to remove unburned carbon adhered on the filter element by oxidation reaction.
Unlike exhaust gas of a diesel engine, regeneration of a porous filter for removing char present in product gas inside a coal gasification furnace requires a safe removal of char containing a few percent of volatile which is specific to coal gas.
Especially, when regenerating a porous filter used in a coal gasification furnace, coal gas containing oxygen is introduced into the porous filter, and the char is ashed (oxidized) by bringing the temperature inside the porous filter up to a prescribed heating temperature. IN order to adopt the regeneration method of Publication 1 in a coal gasification system, there are following problems.
The char present in product gas inside the porous filter used in a coal gasification furnace contains a few percent of volatile. Therefore, if the gas containing 20 vol. % oxygen is introduced as described in Publication 1, due to a large amount of oxygen contained in the gas, combustion of char volatile can cause the explosion in the porous filter during the regeneration process. It becomes impossible to safely regenerate the porous filter.
In the case of the diesel engine, the exhaust gas of high temperature is circulated to regenerate the porous filter. However, in the coal gasification furnace, the operation of the furnace is stopped and gas of comparatively low temperature is used for regenerating the porous filter and thus it is difficult to ash char by using the low temperature gas under a temperature of 80-400° C.